diff --git a/core/mutator.js b/core/mutator.js
index fc9391d3684..08b36533e2b 100644
--- a/core/mutator.js
+++ b/core/mutator.js
@@ -366,8 +366,10 @@ class Mutator extends Icon {
       if (this.block_.saveConnections) {
         const thisRootBlock = this.rootBlock_;
         this.block_.saveConnections(thisRootBlock);
-        this.sourceListener_ = function() {
-          this.block_.saveConnections(thisRootBlock);
+        this.sourceListener_ = () => {
+          if (this.block_) {
+            this.block_.saveConnections(thisRootBlock);
+          }
         };
         this.block_.workspace.addChangeListener(this.sourceListener_);
       }